1. Technical Field
The present invention relates to systems and methods for the detection of surface defects and more particularly to the application of moire interferometry to the detection of surface defects.
2. Discussion
There are a variety of approaches to the inspection of defects on panels of sheet metal or other materials. In general, these approaches attempt to distinguish shape anomalies such as dents, flat spots, creases, or waviness. However, the commonly used inspection techniques for these features is very subjective in nature. For example, a popular dent inspection method, uses an oil-type of highlighting fluid to provide a glossy finish similar to that seen on the finished panel. An inspector then looks at the edge of colored fluorescent tubes placed around the panel as reflected from the surface of the panel. If the inspector detects the presence of a "wave" in the edge of the light as he moves his head past an area, he marks it as having a dent or "error" in the contour.
In such systems, the surface of the panel is acting as a mirror. The light reflected from the panel is deviated according to the surface slope, thereby creating a distorted image of the light fixture. Since the image is always distorted in accordance with the shape of the panel, the inspector is actually looking for some out of the ordinary distortion, such as a sudden change in an area expected to be flat.
There are a number of disadvantages with this type of inspection technique. Both the application of the highlighting fluid and the technique of inspection is time consuming and requires a highly skilled inspector for consistent results; and the technique remains relatively subjective. Also, the oil is messy and must be cleaned off. Markings, surface finish irregularities or an uneven highlighting fluid application, can effect the results. Further, the angle of viewing can become critical. For example, in the case of a car panel, many car owners become aware of irregular contours on the hood of their car when driving under power lines by the way their reflections move over the hood. Different areas of the car are viewed at different angles, relative to the normal to the surface, causing variations in the visibility of such defects. Likewise, for the same reason, the structured light in the highlighting technique is also subject to great sensitivity to viewing angle. Further, since human visual perception is most sensitive to changes from the surrounding area, a dent or flat area seen in one area which has little shape to it, may not be perceived in an area having sharp transitions.
A number of alternative, less subjective techniques have been developed for inspection, which makes use of the measurement of the contour of an object in three dimensions. These approaches include range finding techniques including line of light profiling, stereo viewing, shape from shading, and moire interferometry. While line of light profiling can be simple to analyze, and is largely insensitive to surface reflectivities and background light, it is more suited to providing individual profiles of a surface requiring many video images to fully cover a surface. The sensitivity to a shape anomaly is limited by the video camera resolution.
Stereo viewing requires some surface features to correlate, not necessarily available on sheet metal panels, and can be computer intensive. Shape from shading can capture a full field of information at a time using a grey scale type analysis which is easily executed on modern machine vision systems. However, shading techniques can be fooled by stray lighting, surface reflectivity variations or marks on the panel surface.
Moire interferometry provides full field process data with grey scale values that can be made insensitive to lighting variations, dirt, and other non-shaped parameters. Also, the depth resolution in moire is not limited by the camera resolution as in line of sight systems. However, moire interferometry requires greater demands on the optical system than other methods. Further, prior moire interferometry systems have not been adapted for large panel inspection but instead have been adapted to perform range measurements on small areas. As a result, current moire interferometry systems are generally not large enough to perform analysis on large panel surfaces. In general, conventional moire systems are used as a surface measurement tool, but not as a defect detection tool.
Thus, it would be desirable to provide a panel inspection system which does not require the application of a highlighting fluid, and which is less labor intensive than conventional highlighting techniques. Further, it would be desirable to provide a panel inspection system which does not rely on subjective observations of the inspector. Also, it would be desirable to provide a panel inspection system which provides a means for characterizing defects in an unambiguous quantitative manner.